1. Field of the Invention
The present invention relates to lean NOx traps with improved aging characteristics, and in particular, to lean NOx traps that resist degradation of NO storage capacity upon thermal aging.
2. Background Art
Environmental concerns and governmental regulations have been a continuing impetus for improvements in pollution control from automotive vehicles. The treatment or removal of noxious combustion by-products from the exhausts of such vehicles is a major focus of such efforts. Typically these combustion by-products include incomplete combustion by-products such as carbon monoxide and hydrocarbons. Moreover, the exhausts of these vehicles also include various nitrogen oxides and sulfur oxides. It is desirable and mandated that each of these compounds be reduced to a level that meets certain governmental regulations during vehicle operation.
Lean engines operate under fuel-lean conditions, i.e., a net oxidizing condition. Such an exhaust environment poses unique requirements for treating carbon monoxide, hydrocarbons, and especially NOx. NOx is the most difficult to convert because the exhaust gas is oxidizing yet NOx needs to be reduced in order to convert it to N2. One of the most promising treatments of NOx from lean-burn operation today involves the use of nitrogen oxide absorbent material with precious metals or lean NOx traps. Such NOx trap materials are able to absorb nitrogen oxides during lean-burn operation and to release them when the oxygen concentration in the exhaust gas is lowered. Accordingly, these traps are used with engine systems that operate primarily in a lean air/fuel ratio. However, when it is desired to purge the traps of NOx, the exhaust entering the trap is made fuel richer—either stoichiometric or fuel rich of stoichiometric.
One of the most critical problems for wide application of lean NOx traps (“LNT”) is the thermal stability/durability of the trap. Typically a well made LNT will store and convert NOx to N2 with high efficiency (>90%) when the trap has not been exposed to high temperature (e.g. >750° C.) aging. However, when the trap is aged at high temperature, the trap degradates quickly, losing the high efficiency of storing and converting NOx. In practice, such a high temperature can be expected in the exhaust system where a trap is located, in cases such as high speed/load operation, engine misfire, lean NOx trap desulfation, etc. This relatively poor thermal stability and or durability has hindered the wide application of the LNT trap, or the lean burn engines.
Our studies show that an important mechanism for deactivation of lean NOx traps after high temperature aging is the loss of storage efficiency. And the critical modes for the loss of storage efficiency are the loss of oxidation activity of NO to NO2, and the loss of proximity between the precious metal and the trapping element after thermal aging, thereby limiting the spillover of oxygen atoms from the precious metal to the trapping element to convert the (barium) nitrite to (barium) nitrate, or the NOx species from the precious metal to the trapping element to form nitrates.
Accordingly, there exists a need for lean NOx traps with improved storage efficiency and in particular with improved oxidation activity and proximity of the active components after high temperature aging.